By Dr. Jody Muelaner
A washer is a plate with a hole that lets a fastener to pass through it. The most common shape is a flat disk with a concentric hole. Washers are frequently used to distribute the clamping force from threaded fasteners over a larger area. This prevents bolt heads and nuts from indenting the surfaces of the fastened parts, which could loosen the fastener.
Washers can serve as a spacer and are available in serrated or tabbed to prevent unscrewing. Sprung washers offer axial flexibility and are used to prevent fastening or loosening because of vibration, or to indicate correct pre-load. Several different types of washers are available to suit different applications.
For example, Belleville springs are a sprung washer in the shape of a conical shell, which can be loaded along its axis statically or dynamically. They are also called Belleville or conical spring washers. Belleville washers are typically used in conditions of high current loading or cycling. They are used to absorb vibration and can maintain preload in bolted joints, indicating correct preload based on the gap between adjacent washers. They can also help with “bolt creep” or stress relaxation between a bolt and washer.
However, one feature Belleville washers are unable to provide is significant locking capacity in bolted applications because of a lack of serrations. Although this can be beneficial, depending on the application, because serrations can also damage the clamping surface.
Although they resemble a washer, Belleville washers are often used as a spring to actuate mechanisms and provide suspension. They can be stacked together in different ways to create longer springs with highly configurable spring rates. They have a number of specific advantages for such applications, including:
• Configurability through stacking
• High force within a small or confined space
• Concentric force distribution
Stacks that use a combination of alternating and same direction Belleville springs can achieve particular spring characteristics. For example, when two or more Belleville springs are stacked together, this changes the spring rate. If multiple springs are stacked in the same direction, this is equivalent to arranging springs in parallel. Although the total range of motion remains the same for one spring, the force generated is multiplied by the number of springs.
When Belleville springs are arranged in this way there is also significant friction between the mating surfaces. This creates a damping effect that can reduce vibration.